This topic was opened several times on the forum but each time it got complicated and IMHO failed to produce simple how-to procedure to do this.
So, prerequisite for this procedure is that you have a device capable of adjusting mains delay and applying high pass filtering to to them in addition to providing a separate channel(s) for your subwoofer(s).
Most modern AVR receivers have such possibility, although they often offer to adjust delay only in ft/meters instead of milliseconds. If that is the case you can recalculate ms to feets or meters assuming 1ms equals to 0.343 meters/ms or 1.125ft/ms.
MiniDSP 2x4 HD is also very popular and usefull device for this job.
Note: With both of these devices (AV processor and miniDSP) it is advisable you use LFE input with your sub as that will bypass XO circuitry of your sub, which is fine as we want to have that implemented in AV proc/miniDSP. If LFE input is not available you should dial sub's XO knob to max frequency and set phase knob to 0.
Ok, so this is how you do it. In ideal case your mains should be equally distanced from your LP so time aligning a sub to one main should work for another main too. However, when time aligning a sub to a main I advise you to use the closer main to measure it and another main to play timing reference signal for both sub and main.
So, let's assume we are trying to time align a sub closer to right main, so we'll measure both of them via a band limited sweep using left main to play timing reference signal. As typical XO point used for the sub is around 80Hz band limited sweep of 30-150Hz should do fine. Use sweep length of 2M to allow more time for the steady state to stabilize.
After measuring main and the sub you should apply FDW of 6 cycles to eliminate reflections that blurr the time domain measurements but you shouldn't use "Estimate IR delay". You should unwrap phase in both measurements and use +-360deg buttons to return it to it's original position if it jumps when you unwrap it.
For this eample I used my own main speaker and measurement of F12 Rythmik sub provided by @ernestcarl. Here's how it looks (main in red, sub in blue):
Phase
IR:
Step:
Ok, so what we can see here from overlayed IR and step response is that mains are ahead of sub for app 12ms. As REW doesn't allow you to shift for more than 10ms at a time let's do that first. Select main measurement, press control button and "Offset t=0" button, dial -10 and apply it.
As a result of delaying the main for 10ms, phase, IR and step shifted, and this is what I got:
Phase
IR:
Step:
This looks much better with all 3 graphs. Eyeballing the step graph we can conclude that main still needs to be delayed for a little less than 2ms, so let's use the slider and dial -1.8ms.
And this is what we are now getting:
Phase:
IR:
Step:
So, with total delay of 11.8ms it now looks as sub is perfectly aligned to the main.
What you should do now is configure low pass filter at (say) 80Hz for the sub and equivalent high pass filter for the mains. With AV processors this usualy requires setting your mains to "small". With miniDSP you can choose XO slope, I recommend using LR24 or steeper.
If you have AV processor/receiver this is what you can do and it should sound really good, once you do FR equalisation. Unfortunately I cannot show how phase, IR and step would look with full sweep with the XO and delay implemented this way as I don't have the sub.
If you have miniDSP you can go step further and use rePhase to create a simple FIR filter which will linearise the XO between sub and mains you just created. Assuming you used LR24 at 80Hz this is how it should look, with settings I expect to work with miniDSP 2x4 HD. This should further improve things in time domain around XO so you should be able to get very nice step response with final sweep.
So, prerequisite for this procedure is that you have a device capable of adjusting mains delay and applying high pass filtering to to them in addition to providing a separate channel(s) for your subwoofer(s).
Most modern AVR receivers have such possibility, although they often offer to adjust delay only in ft/meters instead of milliseconds. If that is the case you can recalculate ms to feets or meters assuming 1ms equals to 0.343 meters/ms or 1.125ft/ms.
MiniDSP 2x4 HD is also very popular and usefull device for this job.
Note: With both of these devices (AV processor and miniDSP) it is advisable you use LFE input with your sub as that will bypass XO circuitry of your sub, which is fine as we want to have that implemented in AV proc/miniDSP. If LFE input is not available you should dial sub's XO knob to max frequency and set phase knob to 0.
Ok, so this is how you do it. In ideal case your mains should be equally distanced from your LP so time aligning a sub to one main should work for another main too. However, when time aligning a sub to a main I advise you to use the closer main to measure it and another main to play timing reference signal for both sub and main.
So, let's assume we are trying to time align a sub closer to right main, so we'll measure both of them via a band limited sweep using left main to play timing reference signal. As typical XO point used for the sub is around 80Hz band limited sweep of 30-150Hz should do fine. Use sweep length of 2M to allow more time for the steady state to stabilize.
After measuring main and the sub you should apply FDW of 6 cycles to eliminate reflections that blurr the time domain measurements but you shouldn't use "Estimate IR delay". You should unwrap phase in both measurements and use +-360deg buttons to return it to it's original position if it jumps when you unwrap it.
For this eample I used my own main speaker and measurement of F12 Rythmik sub provided by @ernestcarl. Here's how it looks (main in red, sub in blue):
Phase
IR:
Step:
Ok, so what we can see here from overlayed IR and step response is that mains are ahead of sub for app 12ms. As REW doesn't allow you to shift for more than 10ms at a time let's do that first. Select main measurement, press control button and "Offset t=0" button, dial -10 and apply it.
As a result of delaying the main for 10ms, phase, IR and step shifted, and this is what I got:
Phase
IR:
Step:
This looks much better with all 3 graphs. Eyeballing the step graph we can conclude that main still needs to be delayed for a little less than 2ms, so let's use the slider and dial -1.8ms.
And this is what we are now getting:
Phase:
IR:
Step:
So, with total delay of 11.8ms it now looks as sub is perfectly aligned to the main.
What you should do now is configure low pass filter at (say) 80Hz for the sub and equivalent high pass filter for the mains. With AV processors this usualy requires setting your mains to "small". With miniDSP you can choose XO slope, I recommend using LR24 or steeper.
If you have AV processor/receiver this is what you can do and it should sound really good, once you do FR equalisation. Unfortunately I cannot show how phase, IR and step would look with full sweep with the XO and delay implemented this way as I don't have the sub.
If you have miniDSP you can go step further and use rePhase to create a simple FIR filter which will linearise the XO between sub and mains you just created. Assuming you used LR24 at 80Hz this is how it should look, with settings I expect to work with miniDSP 2x4 HD. This should further improve things in time domain around XO so you should be able to get very nice step response with final sweep.
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